1. Technical Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and particularly to an image forming apparatus and an image forming method using an electrophotographic system.
2. Related Art
In recent years, various image forming apparatuses, such as digital copiers and laser printers, to perform image formation by scanning exposure using a laser light beam and an electrophotographic process have been developed.
The image forming apparatus includes a beam light scanning device to scan the laser light beam onto a photoconductive drum to form an electrostatic latent image on the photoconductive drum. The beam light scanning device includes, for example, a laser oscillator to generate the laser light beam, a polygon mirror to reflect the laser light beam outputted from the laser oscillator toward the photoconductive drum and to scan it onto the photoconductive drum, an f-θ lens and the like.
The electrostatic latent image formed on the photoconductive drum is toner-developed, and the toner-developed image is finally transferred as a recording image to a recording sheet. Accordingly, in order to form a uniform recording image without unevenness, it becomes necessary to form the electrostatic latent image of uniform intensity on the photoconductive drum, and it becomes important to stabilize the intensity of the laser light beam.
However, the intensity of the laser light beam irradiated onto the photoconductive body (photoconductive drum) is not necessarily constant in the beam scanning direction. The main cause is that the transmission loss of the f-θ lens varies according to the incident angle. In general, the incident angle of the laser light beam to the f-θ lens is almost vertical at the center of the f-θ lens, and is obliquely incident on a place close to the end of the f-θ lens. As a result, the transmission loss of the f-θ lens is least at the center, and becomes large toward the end.
This means that from the viewpoint of the intensity of the laser light beam irradiated onto the photoconductive drum, the intensity of the laser light beam is highest at the center of the f-θ lens, and it becomes weak toward the end, and the intensity of the laser light beam becomes uneven in the main scanning direction.
Besides, in a tandem system color image forming apparatus, in the case where a polygon mirror to scan a laser light is used commonly to four colors, a structure is such that the laser light is distributed to photoconductive bodies of respective colors by the mirror, and in addition to the irregularity of the mirror itself, since an incident angle to the mirror varies according to the lasers of the respective colors, even if the same laser light beam power is set, the laser light powers on the drum surfaces of the respective colors are different from each other.
JP-A 2003-320703 or the like discloses a method in which the laser light power of a laser light source is made low at the vicinity of the center of the lens according to the scanning position of the laser, and is made high at the end of the lens, so that the difference in power loss due to the transmissivity cancels out, the laser light power on the surface of the photoconductive drum is uniformed, and the exposure amount is made constant.
In the techniques disclosed in these, light amount correction values corresponding to the scan positions of the laser are prepared, and the adjustment of the laser light power is performed based on the light amount correction values.
Heretofore, with respect to the light amount correction values, the same memory capacity is secured for each of the colors, the absolute amount of the correction values is stored, and a correction circuit of correcting the light amount has a circuit structure of such a system that the value, the absolute amount of the correction value is directly used.
It is necessary that the correction circuit of the light amount processes a large amount of image data at real time, and high speed is required. Thus, it is necessary that the processing from the reading of the correction value to the D/A conversion is processed by the hardware, and a memory (RAM: Random Access Memory) to store these correction values is incorporated in an ASIC (Application Specific Integrated Circuit) or the like or a dedicated high speed RAM is used.
In the conventional system, when the light amount correction is performed, when the resolution of correction (the resolution here means two resolutions of 1) the resolution relating to the number of divided blocks in the main scanning direction, and 2) the resolution relating to the number of bits of a D/A converter used at the correction) is improved, the RAM capacity is increased in proportion thereto. Since picture quality required for an image forming apparatus becomes finer by a recent technical advance, these correction amounts tend to increase, and in the system in which the correction values are stored in all RAMs as they are, the increase of the correction information causes the increase of the RAM capacity, and causes the increase of cost.